Metal film resistors are produced by depositing a thin metal film on a substrate of glass, alumina, oxidized silicon or other insulating substrate. One of the most common resistor materials is a nickel-chromium alloy (Nichrome) or nickel-chromium alloyed with one or more other elements which may be evaporated or sputtered on to a substrate. Nichrome as used here and as used hereafter in this disclosure refers to a nickel-chromium alloy or to nickel-chromium alloyed with one or more other elements. Nichrome is a very desirable thin film because of its stability and near zero TCR's over a relatively broad temperature range (-55.degree. C. to 125.degree. C.). The stability is excellent so long as the sheet resistance is kept below 200 ohms per square on a smooth substrate. Higher ohms per square can be evaporated but are difficult to reproduce causing low yields and exhibit poor stability under high temperature exposure or under operation with voltage applied.
Resistor films are normally stabilized by heating the exposed substrates in an oxidizing ambient to minimize future resistance changes during normal usage. For very thin films, this oxidation causes the resistance of the film to increase as the exposed surfaces of the metal film are oxidized. For thin films approaching discontinuity, this oxidation causes large uncontrollable increases in the final resistance with a corresponding large TCR shift in the positive direction. Operational life tests on these thin film parts invariably fail to meet conventional specifications for stability.
It has been observed that ceramic substrates with "rough" surfaces as measured by a Talysurf profile instrument give higher sheet resistances for a given metal film thickness than "smooth" surfaces. It would be desirable to be able to have a substrate with much rougher surface to use to manufacture in a reproducible manner a resistor with several thousand ohms per square using nichrome or other thin metal film with a stability similar to that exhibited by the thicker or lower sheet resistance films of these materials.
It is therefore the principal object of this invention to produce a high resistance film structure with higher sheet resistance, better stability, and better temperature coefficient of resistance (TCR) than sputtered thin metal film resistors made by well known techniques.
It is a further object of this invention to provide a high resistance film structure which will provide a barrier against possible diffusion of impurities from the substrate into the resistive film.
It is a further object of this invention to provide a method of making a high resistance film structure by modifying the surface of the substrate before the resistive film is applied through the depositing of a relatively rough-surfaced insulating film on the substrate before the resistive film is deposited.
These and other objects will be apparent to those skilled in the art.